The present invention relates to power amplifiers.
In the new EDGE (Enhanced Data Rates for GSM Evolution) modulation scheme for mobile communications networks amplitude, as well as frequency, varies and is part of the modulation. This means that a power amplifier (PA) has to have constant gain rather than constant output power. This is currently the case in GSM (Global System for Mobile Communications) where the amplitude is constant and the information is all in phase.
In the EDGE modulation scheme, since the power amplifier will be non-linear at high output powers both the gain and the phase will be distorted at those high output power.
Of the distortion intermodulation products, the third order (IM3) is the most severe. More information can be found in xe2x80x9cMicrowave Transistor Amplifiersxe2x80x9d by Guillermo Gonzalez, 1984 edition, P.178, ISBN 0-13-581646-7. In EDGE, the IM3 product is likely to be required to be  less than xe2x88x9240 dBc. This is a very difficult specification to meet if a power amplifier is conventionally biassed for linear operation. In the US D-AMPS system the requirement is xe2x88x9228 dBc, and so in D-AMPS even a class AB (very close to class B operation) power amplifier can fulfill the specification. A conventionally biassed EDGE power amplifier would need to be biassed closer to class A to achieve the specification.
However, a class A biassed EDGE power amplifier will also draw current when the amplitude is low during modulation compared to a D-AMPS power amplifier where the current would decrease quite a bit when the lower amplitude is to be transmitted. If the average efficiency is compared then the D-AMPS power amplifier would get almost as high an efficiency as a saturated GSM power amplifier. An EDGE power amplifier would on the other hand have a very low average efficiency.
If the efficiency at maximum output power is considered, however, then an EDGE power amplifier has very high efficiency. The reason that this does not show in the average efficiency is that maximum output power is used only for a very short proportion of the operating time of the power amplifier.
It is therefore desirable to bias the power amplifier in such a way that it can be kept as close to saturation as possible for all output power levels.
There are many known different ways of linearising a power amplifier. Many of these are based on feed back and feed forward at radio frequency and are very sensitive to component variations. U.S. Pat. No. 5,742,201 describes a systems in which the feedback takes place at lower frequency than the radio frequency transmission signal.
It is desirable to provide a bias circuit for a power amplifier which can overcome the disadvantages of the previously considered solutions, with a minimum amount of complexity.
According to one aspect of the present invention, there is provided a circuit for controlling the characteristics of a power amplifier comprising:
first and second inputs;
a first amplitude detection circuit having an input and an output, the input being connected to the first input, and the first detection circuit being operable to produce a first output signal indicative of the amplitude of an input signal received at its input;
a second amplitude detection circuit having an input and an output, the input being connected to the second input, and the second detection circuit being operable to produce a second output signal indicative of the amplitude of an input signal received at its input;
a phase detection circuit having an input and an output, the input being connected to each of the first and second inputs, and being operable to produce a phase signal indicative of the relative phase between input signal supplied to the first and second inputs;
an amplitude control circuit connected to receive the outputs of the first and second amplitude detection circuits and operable to produce an amplitude control signal therefrom; and
a phase control circuit connected to receive the output of the phase detection circuit and operable to produce a phase control signal therefrom.
According to another aspect of the present invention, there is provided a-method of controlling a power amplifier comprising producing a first signal indicative of the amplitude of a high frequency input signal to the amplifier and a second signal indicative of the amplitude of a high frequency output signal from the amplifier, comparing the first and second signals to produce a comparison signal, and supplying the comparison signal to the power amplifier as a bias control signal.
In such a circuit the first amplitude detection circuit preferably includes a diode and a first resistor connected in parallel between the input of the detection circuit and ground, such that the diode is connected in reverse bias between the input and ground, a second resistor connected in series between the input and the output of the detection circuit, and a capacitor connected between the output of the detection circuit and ground.
The second amplitude detection circuit also preferably includes a diode and a third resistor connected in parallel between the input of the detection circuit and ground, such that the diode is connected in reverse bias between the input and ground, a fourth resistor connected in series between the input and the output of the detection circuit, and a capacitor connected between the output of the detection circuit and ground.
The phase detection circuit preferably comprises a diode connected in reverse bias between the input and ground, and wherein the input of the phase detection circuit is connected to the output thereof.
It is emphasised that the term xe2x80x9ccomprisesxe2x80x9d or xe2x80x9ccomprisingxe2x80x9d is used in this specification to specify the presence of stated features, integers, steps or components, but does not preclude the addition of one or more further features, integers, steps or components, or groups thereof.